Apparatus for Producing Secondary Electrons, a Secondary Electrode, and an Acceleration Electrode

ABSTRACT

An apparatus includes a primary electrode and an acceleration electrode. The acceleration electrode or, alternatively, an additional secondary electrode contains a slot that extends obliquely through the acceleration electrode or through the secondary electrode. This measure allows secondary electrons to be produced in a highly effective manner.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of copending U.S. patent applicationSer. No. 10/718,777, filed on Nov. 21, 2003, which claims priority fromGerman Patent Application 102 54 416.6 filed on Nov. 21, 2002, thoseapplications being incorporated herein, by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to an apparatus for producing secondary electrons.The apparatus includes a primary electrode used for producing primaryelectrons, and an acceleration electrode used for accelerating theprimary electrons.

The apparatus is, for example, arranged in a free space used, forexample, for an ion beam to pass through, for holding a plasma and/orfor holding a workpiece or a material to be processed.

The ion beam contains, for example, boron, phosphorus or arsenic ions.In a plasma, electrons are extracted from atoms so that the ions whichare produced in the process have positive charges. Positively chargedions are also referred to as cations.

The primary electrode is, for example, a very thin wire with a verysmall radius of curvature. The radius of curvature results in a coronadischarge. Only charges of the same polarity as the electrode areproduced, because of the corona discharge, at a sufficient distance fromthe primary electrode. This means that the primary electrode has anegative potential in order to produce electrons. By way of example, thediameter of the primary electrode wire is 30 μm (micrometers).

The acceleration electrode is at a different potential than the primaryelectrode, for example at ground potential. The electric field which isformed between the acceleration electrode and the primary electrodeaccelerates the primary electrons toward the acceleration electrode. Theprimary electrodes pass through openings in the acceleration electrode,and retain their direction because of the lack of any electricacceleration field downstream from the acceleration electrode.

By way of example, apparatuses such as these for producing secondaryelectrons are used in ion implantation systems for producing integratedcircuits, in order to counteract charges that can occur on a wafer dueto the cations.

BRIEF SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide an apparatus forproducing secondary electrons, which effectively counteracts charges, oreffectively prevents such charges. Further objects of the invention areto provide a secondary electrode and an acceleration electrode that canbe used in an apparatus for producing secondary electrons.

The invention is based on the idea that the primary electrons havecomparatively high energy, for example, several hundred electron volts(eV), because of the acceleration voltage. The high energy makes itpossible, for example, for the primary electrons which reach asemiconductor wafer directly to produce a strong negative charge there,which is likewise not wanted. For example, the primary electrons can becarried by an ion beam. In order to prevent such processes or similarprocesses, in a first embodiment of the apparatus, the accelerationelectrode contains at least one aperture opening, which extendsobliquely through the acceleration electrode. The primary electrons thusarrive at a side wall of the aperture opening before they leave theapparatus and, for example, reach free space. Furthermore, when theprimary electrons arrive, they produce secondary electrons which havecomparatively low energy. In addition, the aperture opening has agreater length than the material thickness of the electrode, since it isat an inclined angle.

Based on the first embodiment of the apparatus, a secondary electrode isconstructed to perform the function of an acceleration electrode inaddition to other functions. The secondary electrode has threefunctions:

producing the acceleration voltage;

producing secondary electrons; and

interrupting the direct path for primary electrons to the exterior, forexample, to free space.

In a second embodiment of the invention, the apparatus for producingsecondary electrons contains a separate acceleration electrode foraccelerating the primary electrons emitted by a primary electrode. Inthis case, the secondary electrode produces secondary electrons whenaccelerated primary electrons arrive. The secondary electrode ispreferably arranged on the side of the acceleration electrode whichfaces away from the primary electrode. The secondary electrode is thuslocated between the acceleration electrode and free space. Primaryelectrons arrive at the secondary electrode, and produce secondaryelectrons. Because the secondary electrode is physically separated fromthe acceleration electrode, the secondary electrons pass to theexterior, for example to free space, without being disturbed by theelectric field for producing and accelerating the primary electrons.Since the secondary electrode is also located on the same side of freespace as the primary electrode, the number of primary electrons whichwill reach free space is reduced.

In one development based on the second embodiment of the apparatus, thesecondary electrode contains at least one aperture opening, whichextends obliquely through the secondary electrode. This development isbased on the same ideas as the first embodiment of the invention. Thesecondary electrode has two functions:

producing the secondary electrons when primary electrons arrive; and

interrupting the direct path for primary electrons to the exterior, forexample to free space.

Other measures in order to use the aperture opening to impede thepassage of primary electrons or to prevent them from passing through mayalso be taken instead of or in addition to an aperture opening whichextends obliquely through the acceleration electrode or through thesecondary electrode. For example, an electrode having an apertureopening which runs in the normal direction to the electrode surface canbe inclined with respect to free space or to the ion beam or can bearranged with a specific offset from the aperture opening and primaryelectrode, in order to prevent primary electrons from passing throughand thus to achieve the same effects as those explained above.

In one development which is applicable to both aspects, the apertureopening is an elongated hole which preferably extends parallel to theprimary electrode or to a wire of the primary electrode. The elongatedhole can be produced in a simple manner, for example, using a millingmachine. Oblique elongated holes can also be produced without anyproblems by using a milling machine. Alternatively, laminates are used.

In one refinement, the side surfaces of the aperture opening overlap oneanother or are aligned with the arrival direction at which the majorityof primary electrons reach the aperture opening. An aligned arrangementensures that, on the one hand, no primary electrons can reach theexterior, for example, free space, directly, and that, on the otherhand, the impact surface in the aperture opening is large in comparisonto the length of the aperture opening. In consequence, a large number ofsecondary electrons can pass to the exterior, for example, to freespace, through the aperture opening in a simple manner.

In one development, the aperture opening is arranged inclined at anangle of between 30° and 70° to the surface normal of the electrode inthe area of the aperture opening. The appropriate angle depends on thethickness of the electrode. For example, an angle of 55° is particularlysuitable for a thickness of about 5 mm. In one refinement, the angle isdefined by:

tan(90°−w)=d/b,

where w is the angle, d is the thickness of the electrode with theaperture opening, and b is the width of the aperture opening.

In one refinement, the distance between the side walls of the apertureopening is between 2 mm and 6 mm. The appropriate distance once againdepends on the thickness of the electrode with the aperture opening. Forexample, a distance between the side walls of 4 mm is particularlysuitable for an electrode thickness of about 5 mm and an inclinationangle of 55°.

In another development, only one aperture opening is provided for eachprimary electrode. This results in a physically simple apparatus. In onerefinement, the apparatus contains only one primary electrode. Thismeasure further simplifies the configuration of the apparatus.

Alternatively, however, the apparatus contains two or more primaryelectrodes, each of which has at least one associated aperture opening.The use of aperture openings with different aperture directions meansthat secondary electrons which are produced by the primary electronsfrom different primary electrodes can be concentrated at a specificpoint outside the apparatus, for example, in an electron cloud in freespace. While the physical length and the neutralization effect remainthe same, this ensures that fewer primary electrons can, for example,reach the impact location of the ion beam, that is to say, for example,the semiconductor wafer or a material to be irradiated.

In another development, aluminum, an aluminum alloy, graphite, agraphite compound or aluminum oxide Al₂O₃ is used as the material forthe electrode with the aperture opening. Aluminum Al and graphite C havea yield of 1, or else of the order of magnitude of 1, with an energy of300 eV. This means that every primary electron that arrives produces asecondary electron. The yield is also referred to as the secondaryelectron emission coefficient. The secondary electron emissioncoefficient of aluminum oxide is at least twice as great as that ofaluminum or graphite.

A roughened surface on the electrode with the aperture opening leads togreater secondary electron emission. In one refinement, the meanroughness of the surface is 6.3 μm.

In another development with a separate acceleration electrode and asecondary electrode, the acceleration electrode contains more than 100openings, more than 500 openings, or even more than 1000 openings. Theopenings are, for example, holes with a diameter of about 1 mm. However,wire meshes may also be used as an alternative.

In one development, aluminum or an aluminum alloy is used as thematerial for the acceleration electrode. In one refinement, aluminumwith a purity of 99.9% or more than 99.9% is used. This material isparticularly suitable for use as the electrode material for producingintegrated circuit arrangements. The mean surface roughness of theacceleration electrode in one refinement is 4.3 μm, that is to say theacceleration electrode is smoother than the secondary electrode.

In one development, the primary electrode or the primary electrodes isor are arranged parallel to the propagation direction of the ion beam.This type of arrangement is used in particular for so-calledhigh-current ion implantation systems. The ion beam current in thesesystems is greater than 1 mA. For example, the ion current is in therange between 1 and 25 mA. In one development, the primary electrodesmay, for example, be arranged around the ion beam.

In an alternative development, the primary electrode or the primaryelectrodes is or are arranged transversely with respect to thepropagation direction of the ion beam, preferably at an angle of 90°.This arrangement is particularly suitable for systems in which the ionbeam is, for example, guided in the form of a scanning movement, that isto say it is scanned. A scanning movement such as this is used inparticular for medium-current ion implantation systems, in which the ioncurrent has values of less than 1 mA. However, high-current ionimplantation systems also exist in which the ion beam carries out ascanning movement.

The invention also relates to a secondary electrode having the featuresof the secondary electrodes mentioned above, that is to say inparticular with an aperture opening which extends obliquely through thesecondary electrode. The technical effect described above thus alsoapplies to the secondary electrode.

The invention additionally relates to an acceleration electrode which,in particular, has the features of the acceleration electrode mentionedabove. In particular, the acceleration electrode contains more than 100openings, more than 500 openings or more than 1000 openings. Thetechnical effects mentioned above thus likewise apply to thisacceleration electrode.

By way of example, the inventive apparatus or the inventive electrodecan be used in a simple manner in an ion implantation system of theEATON implanter type, in particular in a system of the NV 8250 P type.The dimensions stated below relate to this type of system. However,other types of systems can also be modified in a similar way in order touse the invention.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an apparatus for producing secondary electrons, in particular asecondary electrode and an acceleration electrode, it is neverthelessnot intended to be limited to the details shown, since variousmodifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a view of an apparatus for producing secondary electrons usinga corona discharge;

FIGS. 2A and 2B are views of a secondary electrode; and

FIG. 3 is a plan view of an acceleration electrode.

FIG. 4 is a view of the apparatus for producing secondary electronsusing a corona discharge, showing the primary electrode configuredparallel to the ion beam.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a configuration of anelectron production apparatus 10 which contains an aperture area 12 or achamber for an ion beam 14 to pass through. The ion beam 14 is producedusing non-illustrated units of an ion implantation system, specificallyby using an ion source, for example, an RF (radio frequency) source, amass separator arrangement, for example, a magnet, and an accelerationpath.

Before or after passing through the aperture area 12, the ion beam 14 isdeflected, for example, using a deflection system, so that it carriesout a scanning movement.

The aperture area 12 is bounded at the bottom by a shielding pivotingelectrode 16. The pivoting electrode 16 is mounted such that it canpivot about a point P. When the production apparatus 10 is in a normalmode of operation, the pivoting electrode 16 is in the positionillustrated in FIG. 1, that is to say the ion beam 14 can pass throughthe aperture area 12 without any impediment. Conversely, in a testoperating mode, the pivoting electrode 16 is pivoted upwards into theaperture area 12, so that the ion beam 14 can no longer strike a wafer18.

The aperture area 12 is bounded at the top by a secondary electrode 20which extends parallel to the pivoting electrode 16 in the normaloperating mode. The configuration of the secondary electrode 20 will beexplained in more detail below.

A primary electron production apparatus 26, which is used for producingprimary electrons, is arranged a short distance away on the outside ofthe secondary electrode 20. The electron production apparatus 26contains three glow wires 28, 30 and 32, which are arranged in a rowparallel to one another, for example, with uniform spacings betweenthem. The glow wires 28, 30 and 32 run at an angle of 90° with respectto the propagation direction of the ion beam 14. The glow wires 28, 30and 32 are surrounded by a triple reflector 34, which has three mutuallyseparated cutouts 36, 38 and 40. A respective glow wire 28, 30 and 32extends in the center of each cutout 36, 38, and 40. An accelerationelectrode 42, which lies in a plane, is, for example, a grating formedby holes or a wire mesh. The acceleration electrode 42 is locatedopposite the openings of the cutouts 36, 38, and 40 and at a shortdistance from the triple reflector 34.

The pivoting electrode 16, the secondary electrode 20 and theacceleration electrode 42 are, in the exemplary embodiment, composed ofaluminum. The glow wires 28 to 32 are composed, for example, oftungsten. Aluminum is likewise suitable as the material for the triplereflector 34.

The secondary electrode 20 contains three slots 50, 52 and 54 which arearranged obliquely in the secondary electrode 20 and are adjacent to theglow wires 28, 30 and 32, in this sequence. The slots 50 and 52 are thuslocated closer to the point at which the ion beam 14 enters the aperturearea 12 than the slot 54. The side walls of the slots 50 and 52 areparallel to one another. The side walls of the slots 50 and 52 thusdefine an angle of −60° with respect to a normal to the surface of thesecondary electrode 20 which faces away from the aperture area 12. Inthis case, minus means that the angle from the normal runs in acounterclockwise direction. The side walls of the slot 54 are parallelto one another, but are inclined in a direction which opposes theinclination direction of the side walls of the slots 50 and 52. The sidewalls of the slot 54 thus define an angle of +60° with respect to thenormal to the surface of the secondary electrode 20 which faces awayfrom the aperture area 12. The thickness of the secondary electrode 20and the angle are of such magnitudes that pointed edges of the slots 50,52 and 54 are each aligned with one another in the direction of thenormal to the surface of the secondary electrode 20. Inlet openings inthe slots 50, 52 and 54 are arranged opposite the glow wires 28, 30 and32, in this sequence.

A potential of −300 V (volts) is applied to the triple reflector 34 andto the glow wires 28 to 32. A voltage which is superimposed on thispotential, in particular a DC voltage of, for example, 30 volts, leadsto a current flow which causes the glow wires 28 to 32 to glow. Then, byway of example, 5 A (amperes) per glow wire 28 to 32 flows through theglow wires 28 to 32. There is a potential of 0 V on the pivotingelectrode 16, on the secondary electrode 20 and on the accelerationelectrode 42. Thus, for example, the primary electrons are acceleratedfrom the glowing filament 28 to the side surface of the slot 50 which islocated closer to the entry point of the ion beam 14. As a result ofpassing through the acceleration electrode 42, these primary electrons60 arrive at this side surface, where each of them produces a secondaryelectron. The second electrons 62 which are produced in this way areattracted by the ion beam 14 and, because of the directional effect ofthe slot 50, arrive in an electron cloud 64 on a path that is inclinedwith respect to the propagation direction of the ion beam.

Primary electrons 66 pass in the same way from the glow wire 30 throughthe acceleration electrode 42 to the side surface of the slot 52 whichis closer to the entry point of the ion beam 14. The primary electrons66 produce secondary electrons 68, which are attracted by the ion beam14, and enter the electron cloud 64.

Primary electrons 70 are accelerated from the glow wire 32 towards theacceleration electrode 42. The primary electrons 70 pass through theacceleration electrode 42 and reach the side surface of the slot 54which is further away than the point at which the ion beam 14 enters theaperture area 12. The primary electrons 70 produce secondary electrons72, which likewise enter the electron cloud 64. While the secondaryelectrons 62 and 68 mainly enter the electron cloud 64 parallel to oneanother, the secondary electrons 72 are in the opposite direction to thesecondary electrons 62 and 68, because of the different inclinationdirection of the slot 54. Primary electrons 70 which pass randomlythrough the slot 54 are also directed away from the wafer 18 due to theinclination of the slot 54.

FIG. 2A shows a secondary electrode 100 for an EATON implanter of theNV-8250 type, that is to say in particular for 6-inch waferimplantations or, after conversion, for 8-inch wafer implantations. Thesecondary electrode 100 is used instead of the secondary electrode 20 inan electron production apparatus which contains only one glow wire. Aslot 102 which is arranged in the secondary electrode 100 corresponds,for example, to the slot 50, 52 or 54. The secondary electrode 100 has arectangular base body, on whose longitudinal faces two attachment webs104 and 106 are arranged centrally. The attachment webs 104 and 106 havea length which corresponds approximately to ⅓ of the length of the basebody of the secondary electrode 100. The attachment web 104 projectsabout 10 mm beyond the base body of the secondary electrode 100. Incontrast, the attachment web 106 projects only about 3 mm beyond thebase body of the secondary electrode 100. The base body of the secondaryelectrode 100 has length of 162 mm and a width of about 36 mm. Anopening of the slot 102 extends close to the attachment web 106,virtually over the entire length of the base body. In the exemplaryembodiment, the length of the slot 102 without including rounded areasat the ends of the slot 102 is 150 mm.

FIG. 2A also shows a section plane A in the center of the secondaryelectrode 100. The section plane A is located transversely with respectto the longitudinal axis of the secondary electrode 100.

FIG. 2B shows the secondary electrode 100 along the cross section lyingin the section plane A. The thickness of the secondary electrode 100 isabout 5 mm. As is illustrated in FIG. 2B, side walls 110 and 112 of theslot 102 are inclined at an angle of −55° with respect to a normal N.The distance between the side walls 110 and 112 is 4 mm.

The secondary electrode 100 is composed of aluminum, that is to say, ofthe material Al 99. The mean roughness of the surface is 6.3 μm.

FIG. 3 shows an acceleration electrode 150 which is used instead of theacceleration electrode 42 in the same system in which the secondaryelectrode 100 is also used. The acceleration electrode 150 has a lengthof 162 mm and a width of 11 mm. A large number of holes 160 to 166, forexample, more than 500 holes, are located within a frame 152. The holes160 to 166 each have a diameter of 1 mm, and are arranged in six rows170 to 180 which run parallel to one another. The holes 160, 162 of twoadjacent rows 170, 172 are offset by 0.65 mm with respect to oneanother. The distances between the inner rows 174 and 176, between thecentral rows 172, 178 and between the outer rows 170 and 180 are 1.13mm, 3.38 mm and 5.63 mm, respectively. Mutually adjacent holes 164, 166within one row 170 to 180 each have a separation of 1.3 mm between thecenter points of the holes 164 and 166.

The acceleration electrode 150 is illustrated greatly enlarged in FIG.3. The acceleration electrode 150 is composed of aluminum Al 99.9, sothat only a small number of stray atoms emerge from the accelerationelectrode 150. Implantation processes can thus be carried out with ahigh yield. The mean roughness of the surface is 4.3 μm. The thicknessof the frame 152 is 1.5 mm. The acceleration electrode 150 has athickness of 1 mm within the frame 152.

In another exemplary embodiment, the electron production apparatus 10has no acceleration electrode 42. The functions of the accelerationelectrode 42 are then additionally carried out by the secondaryelectrode 20.

In a further exemplary embodiment, laminates which, for example, areheld in a frame are used instead of the oblique slots. Two adjacentlaminates each form one aperture opening, which extends obliquelythrough the frame. Each laminate is an elongated thin metal platelet,for example, composed of aluminum.

1. An apparatus for producing secondary electrons, comprising: at leastone primary electrode for producing primary electrons; an accelerationelectrode for accelerating the primary electrons; a secondary electrodefor producing secondary electrons when the accelerated primary electronsarrive; and there being a potential of 0 V on the secondary electrodeand on the acceleration electrode.
 2. The apparatus according to claim1, wherein: said secondary electrode is formed with at least oneaperture opening; said aperture opening extends obliquely through saidsecondary electrode and/or said aperture opening prevents primaryelectrons from passing through.
 3. The apparatus according to claim 2,wherein said aperture opening is formed by an elongated hole defined byside surfaces configured parallel to one another.
 4. The apparatusaccording to claim 2, wherein: said aperture opening is defined by sidesurfaces that overlap in a direction at which the primary electronsarrive into said aperture opening.
 5. The apparatus according to claim2, wherein: said secondary electrode has a surface in which saidaperture opening is formed; said surface has a normal; and said sidesurfaces of said aperture opening are configured aligned with saidnormal.
 6. The apparatus according to claim 2, wherein said apertureopening is formed by laminates.
 7. The apparatus according to claim 1,wherein: said secondary electrode is formed with at least one apertureopening having an aperture direction configured at an angle of amagnitude of between 30° and 70° with respect to a normal of saidsecondary electrode near said aperture opening.
 8. The apparatusaccording to claim 7, wherein: said angle has a magnitude of 55° withrespect to the normal of said secondary electrode near said apertureopening.
 9. The apparatus according to claim 1, wherein: said secondaryelectrode is formed with at least one aperture opening having anaperture direction configured at an angle defined by: tan(90°−w)=d/b;and w is said angle, d is a thickness of said secondary electrode, and bis a width of said aperture opening.
 10. The apparatus according toclaim 1, wherein: said secondary electrode has side walls defining anaperture opening; said side walls are spaced a distance apart; and saiddistance between said side walls is between 2 mm and 6 mm.
 11. Theapparatus according to claim 10, wherein said distance between said sidewalls is 4 mm.
 12. The apparatus according to claim 1, wherein: said atleast one primary electrode includes only one primary electrode.
 13. Theapparatus according to claim 1, further comprising: a plurality ofprimary electrodes; said secondary electrode formed with a plurality ofaperture openings; each one of said plurality of primary electrodesassociated with a respective one of said plurality of aperture openings;and at least one of said plurality of aperture openings being at adifferent inclination angle than another one of said plurality ofaperture openings.
 14. The apparatus according to claim 1, wherein: saidsecondary electrode is formed with at least one aperture opening; saidsecondary electrode is made of aluminum or of an aluminum alloy.
 15. Theapparatus according to claim 1, wherein: said secondary electrode ismade of Al 99 or of an even purer aluminum.
 16. The apparatus accordingto claim 1, wherein: said secondary electrode is formed with at leastone aperture opening; and said secondary electrode is made of graphiteor contains at least 60% by mass of graphite.
 17. The apparatusaccording to claim 1, wherein: said secondary electrode is formed withat least one aperture opening; and said secondary electrode is made ofaluminum oxide.
 18. The apparatus according to claim 1, wherein: saidsecondary electrode is formed with at least one aperture opening; andsaid secondary electrode has a mean surface roughness of between 5 and 8μm.
 19. The apparatus according to claim 1, wherein: said accelerationelectrode is formed with at least 100 openings.
 20. The apparatusaccording to claim 1, wherein: said acceleration electrode is formed atleast 500 openings.
 21. The apparatus according to claim 1, wherein:said acceleration electrode is formed with at least 1000 openings. 22.The apparatus according to claim 19, wherein: said accelerationelectrode includes a wire mesh having at least 100 holes or meshes. 23.The apparatus according to claim 1, wherein: said acceleration electrodeis formed with at least 100 openings; and said acceleration electrode ismade of aluminum or an aluminum alloy.
 24. The apparatus according toclaim 1, wherein: said acceleration electrode is formed with at least100 openings; and said acceleration electrode is made of Al 99.9 or aneven purer aluminum.
 25. The apparatus according to claim 1, wherein:said acceleration electrode is formed with at least 100 openings; andsaid secondary electrode has a mean surface roughness; and saidacceleration electrode has a mean surface roughness of less than saidmean surface roughness of said secondary electrode.
 26. The apparatusaccording to claim 25, wherein: said mean surface roughness of saidacceleration electrode is between 2.5 and 6 μm.
 27. The apparatusaccording to claim 1, further comprising: a free space for an ion beamto pass through; and a workpiece; said ion beam being directed at saidworkpiece.
 28. The apparatus according to claim 27, wherein saidworkpiece is a semiconductor substrate.
 29. The apparatus according toclaim 1, further comprising: a free space used for holding a material orworkpiece to be processed.
 30. The apparatus according to claim 1,further comprising: a free space for an ion beam to pass through; saidprimary electrode configured parallel to a propagation direction of saidion beam.
 31. The apparatus according to claim 1, further comprising: afree space for an ion beam to pass through; said primary electrodeconfigured transversely with respect to a propagation direction of saidion beam.